1. Field of the Invention
The present invention relates to tripentyl citrates having an optionally acylated, preferably acetylated, OH group, a process for the preparation of the tripentyl citrates and the use of the tripentyl citrates as plasticizers.
2. Description of the Related Art
Polyvinyl chloride (PVC) is among the most economically important polymers. It is widely used both in the form of rigid PVC and in the form of flexible PVC.
To produce a flexible PVC, plasticizers are added to the PVC. In the majority of instances phthalic esters are used, particularly di-2-ethylhexyl phthalate (DEHP), diisononyl phthalate (DINP) and diisodecyl phthalate (DIDP).
Discussions about reproductive toxicity effects have in some instances already led to an increased level of identification marking under hazardous materials legislation and have also led to limitations on the use of certain phthalate plasticizers in toys for toddlers, and it therefore has to be assumed that the use of these phthalates will reduce markedly in the future, particularly in sensitive applications, such as food-or-drink packaging and medical applications. There is therefore a need for plasticizers which are not subject to identification-marking requirements and which can be used for example as DEHP replacement, and which are prepared from raw materials of which large quantities are available worldwide.
One conceivable alternative is the use of plasticizers based on citric acid. In particular, the best-known member of this class of compound, acetyl tri-n-butyl citrate (ATBC), has been increasingly used for the production of children's toys from flexible PVC, not least since certain phthalates have been subject to the abovementioned restriction on use. Another supporting factor here is the view taken by the Scientific Committee for Toxicology, Ecology and Ecotoxicology (CSTEE), which was an EU committee of toxicology experts, according to which the use of this plasticizer in flexible PVC toys is risk-free even for toddlers.
However, it is known that ATBC has higher volatility and migration rate than, for example, DEHP and therefore still has potential for optimization. There has therefore been no lack of attempts to develop structurally varied citric esters in which these disadvantages have been eliminated. In principle, this can be achieved via use of longer-chain alcohols for the esterification reaction. Examples of compounds which have long been known and also marketed are therefore acetyl tri-2-ethylhexyl citrate (ATEHC) or butyryl tri-n-hexyl citrate (BTHC). The preparation of ATBC is described by way of example in WO 03/008369, the entire contents of which are incorporated by way of reference into this application.
Alongside these, citrate esters having a free, i.e. non-acylated, OH group have also been described. By way of example, EP 1 063 257 mentions trialkyl esters of citric acid where alkyl=C6 to C10, the alkyl chains preferably being linear. When compared with their carboxylated analogues, these generally feature improved efficiency and gelling, but also feature poorer thermal stability.
Alongside the esters of citric acid or of acetylated citric acid with only one alcohol, such as butanol (i.e. ATBC) or 2-ethylhexanol (i.e. ATEHC), there also exist esters based on alcohol mixtures having different numbers of carbon atoms. Schär et al. describe, in Ip.com Journal (2004), 4 (8), pp. 15 et seq., the use of citric esters (having a free or derivatized OH group) based on alcohol mixtures which are composed of at least two different alcohols in the range from C2 to C22 and in which the alcohols are specifically mixed prior to the esterification reaction.
EP 1 256 566 describes mixtures of citric esters whose alkyl chains are composed of a certain percentage of butyl and a complementary percentage of longer radicals.
However, citric esters based on linear alcohols (C6 and higher) are generally relatively expensive, since the alcohols have to be prepared via ethylene oligomerization or by way of fatty acid hydrogenation or hydrogenation of the fatty acid esters, e.g. methyl ester, whereas the use of competitively priced alcohols is a precondition for large-scale industrial production on the multiple-thousand-tonne scale. Longer-chain esters for example with C8 alcohols have very low volatility but exhibit gelling which is too slow for certain plastisol-processing techniques. The relatively low efficiency is attended by a need to add relatively large amounts, and this generally contributes to a further increase in the cost of the formulation and to an increase in the amount of raw material consumed.